Science Flashcards

During an action potential, which sequence of ion channel events correctly describes what happens immediately after the membrane potential reaches its peak depolarization?

Answer: Voltage-gated sodium channels inactivate while voltage-gated potassium channels open

At peak depolarization, voltage-gated Na⁺ channels rapidly inactivate (enter an inactivation gate state), preventing further Na⁺ influx. Simultaneously, voltage-gated K⁺ channels — which open more slowly — reach full opening, allowing K⁺ to rush out of the cell. This K⁺ efflux drives repolarization. The Na⁺/K⁺ pump restores ion gradients only after repolarization is complete and operates too slowly to cause immediate repolarization.

A sealed container holds a mixture of nitrogen gas (N₂) and hydrogen gas (H₂) at equilibrium: N₂(g) + 3H₂(g) ⇌ 2NH₃(g). If the volume of the container is suddenly halved at constant temperature, what happens to the equilibrium position?

Answer: It shifts right, producing more NH₃

By Le Chatelier's Principle, decreasing volume increases pressure. The system responds by shifting toward the side with fewer moles of gas to reduce pressure. The left side has 1 + 3 = 4 moles of gas; the right side has 2 moles. Shifting right (toward NH₃) reduces total moles of gas, partially relieving the increased pressure. Temperature constancy is irrelevant to the pressure-volume effect on equilibrium position.

Which of the following best explains why C4 plants such as corn outcompete C3 plants such as wheat in hot, dry environments?

Answer: C4 plants use a carbon-concentrating mechanism that suppresses photorespiration by delivering CO₂ directly to RuBisCO

In C4 plants, CO₂ is first fixed in mesophyll cells into 4-carbon compounds (oxaloacetate), then shuttled to bundle-sheath cells where it is released at high concentration around RuBisCO. This spatial separation maintains a high CO₂-to-O₂ ratio at the active site of RuBisCO, virtually eliminating the wasteful oxygenase reaction (photorespiration). C3 plants suffer significant photorespiration losses in hot conditions when stomata close and O₂ accumulates. CAM plants — not C4 — fix carbon at night.

A woman who is a carrier for red-green color blindness (X-linked recessive) has children with a man who has normal color vision. What is the probability that their daughter will be color blind?

Answer: 0%

The mother is a carrier: genotype X^B X^b. The father has normal vision: X^B Y. Daughters receive one X from each parent. From the mother, a daughter receives either X^B (normal) or X^b (carrier allele); from the father, all daughters receive X^B. Possible daughter genotypes: X^B X^B (normal) or X^B X^b (carrier). Neither genotype is X^b X^b (color blind), because the father contributes only the dominant X^B allele to daughters. Therefore, 0% of daughters will be color blind, though 50% will be carriers.

The half-life of carbon-14 is approximately 5,730 years. A fossilized wood sample is found to contain only 12.5% of the carbon-14 activity compared to living wood. Approximately how old is the sample?

Answer: 17,190 years

Each half-life reduces the remaining ¹⁴C by half. Starting at 100%: after 1 half-life → 50%; after 2 half-lives → 25%; after 3 half-lives → 12.5%. Three half-lives have elapsed: 3 × 5,730 = 17,190 years. This is a straightforward application of exponential decay — each step divides the remaining fraction by 2.

Two objects of different masses are dropped simultaneously from the same height in a vacuum. Compared to the more massive object, the less massive object will:

Answer: Strike the ground at exactly the same time

In a vacuum there is no air resistance. Gravitational acceleration g ≈ 9.8 m/s² acts equally on all objects regardless of mass, as Galileo demonstrated and Newton formalized: F = mg, so a = F/m = g. Both objects experience the same acceleration and fall identically. The distractor about 'less inertia' is incorrect — although a lighter object has less inertia, gravity also exerts proportionally less force on it, so the two effects cancel perfectly. The air resistance option would apply in air, not a vacuum.